What causes Gravity?

One of Einstein's old tutors, a man by the name of Minkowski showed that the special
theory of relativity could be expressed in an interesting way.

The world we live in consists of four dimensions, the three space dimensions and one
that is not exactly time but is related to time (it is in fact time multiplied by the
square root of -1). This is not at all easy to understand but it means that
space-time as we call it has some rather weird properties. In particular, when you
move through one of the space dimensions you also travel, unwittingly, through time.
You do not notice this, indeed as far as you are concerned nothing happens to you at all,
but someone observing you would say that you have travelled through time. Of course,
we are always travelling through time, but when you travel through space you travel
through time by less that you expect. The most famous example of this effect is the
"Twins Paradox".

All the effects of special relativity, such as the slowing down of clocks and the
shrinking of rods follow from the above. In fact, it is often better to think of
some things, such as electromagnetic fields as being four-dimensional objects.
However, the important thing to remember for the moment, is: when you move through space
you are compelled to move through time but, when you move through time (which of course
you are always doing) you do not have to move through space.

So, what does this have to do with gravity? It is quite simple! When a mass
is present in the above space-time it distorts it so that whilst it remains true that
travelling through space causes you to travel through time, travelling through time now
causes you to move (accelerate) through space. In other words just by existing, you
are compelled to move through space - this is gravity.

The particular advantage of this theory of gravity (General Relativity) is that it
explains, at a stroke, all the observed properties of gravity. For example the fact
that it acts equally on all objects and substances becomes obvious when you thing of
gravity as a distortion of space-time rather than a force.

Imagine that you are in free space, away from any planets or stars, when suddenly a
planet is created quite close to you. You would not be aware that anything is
happening to you, you would feel no force, but you would find that you started to
accelerate towards the planet. This is just like the case where you travel through
space, you are not aware that you have also travelled through time but people observing
you are.

Can you feel gravity?

You can argue that we do not. What happens is that, as you pass through time, the
distortion of space-time caused by the presence of the earth accelerates your body towards
the centre of the earth. However, when your feet are touching the ground, the ground
exerts a force on your feet in an upwards direction which pushes you in the opposite
direction. In other words you are being accelerated upwards with respect to
space-time by the force of the ground acting on your feet. It is exactly the same as
the force which seems to push you back in your car seat when you accelerate, what is
really happening is that the seat is pushing you forwards.